The present invention pertains to improvements to a burner as for a Stirling cycle heat engine and more particularly to improvements relating to control of the fuel and air input provided to the burner.
Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression. In an ideal Stirling thermodynamic cycle, the working fluid undergoes successive cycles of isovolumetric heating, isothermal expansion, isovolumetric cooling and isothermal compression. Practical realizations of the cycle, wherein the stages are neither isovolumetric nor isothermal, are within the scope of the present invention and may be referred to within the present description in the language of the ideal case without limitation of the scope of the invention as claimed.
Additional aspects of Stirling cycle machines and improvements thereto are discussed in a co-pending U.S. patent application Ser. No. 09/517,245, filed Mar. 2, 2000, and incorporated herein by reference.
The principle of operation of a Stirling cycle engine is readily described with reference to FIGS. 1a-1f, wherein identical numerals are used to identify the same or similar parts. Many mechanical layouts of Stirling cycle engines are known in the art, and the particular Stirling engine designated generally by numeral 10 is shown merely for illustrative purposes. In FIGS. 1a to 1d, a piston 12 (otherwise referred to herein as a xe2x80x9ccompression pistonxe2x80x9d) and a second piston (also known as an xe2x80x9cexpansion pistonxe2x80x9d) 14 move in phased reciprocating motion within cylinder 16. Compression piston 12 and expansion piston 14 may also move within separate, interconnected, cylinders. Piston seals 18 prevent the flow of a working fluid contained within cylinder 16 between piston 12 and piston 14 from escaping around either piston 12. The working fluid is chosen for its thermodynamic properties, as discussed in the description below, and is typically helium at a pressure of several atmospheres. The volume of fluid governed by the position of expansion piston 14 is referred to as expansion space 22. The volume of fluid governed by the position of compression piston 12 is referred to as compression space 24. In order for fluid to flow between expansion space 22 and compression space 24, whether in the configuration shown or in another configuration of Stirling engine 10, the fluid passes through regenerator 26. Regenerator 26 is a matrix of material having a large ratio of surface area to volume which serves to absorb heat from the working fluid when the fluid enters hot from expansion space 22 and to heat the fluid when it passes from compression space 24 returning to expansion space 22.
During the first phase of the engine cycle, the starting condition of which is depicted in FIG. 1a, piston 12 compresses the fluid in compression space 24. The compression occurs at a constant temperature because heat is extracted from the fluid to the ambient environment. In practice, a cooler 68 (shown in FIG. 2) is provided, as will be discussed in the description below.
The condition of engine 10 after compression is depicted in FIG. 1b. During the second phase of the cycle, expansion piston 14 moves in synchrony with compression piston 12 to maintain a constant volume of fluid. As the fluid is transferred to expansion space 22, it flows through regenerator 26 and acquires heat from regenerator 26 such that the pressure of the fluid increases. At the end of the transfer phase, the fluid is at a higher pressure and is contained within expansion space 22, as depicted in FIG. 1c. 
During the third (expansion) phase of the engine cycle, the volume of expansion space 22 increases as heat is drawn in from outside engine 10, thereby converting heat to work. In practice, heat is provided to the fluid in expansion space 22 by means of a heater 64 (shown in FIG. 2) which is discussed in greater detail in the description below. At the end of the expansion phase, the hot fluid fills the full expansion space 22 as depicted in FIG. 1d. During the fourth phase of the engine cycle, the fluid is transferred from expansion space 22 to compression space 24, heating regenerator 26 as the fluid passes through it. At the end of the second transfer phase, the fluid is in compression space 24, as depicted in FIG. 1a, and is ready for a repetition of the compression phase. The Stirling cycle is depicted in a P-V (pressure-volume) diagram as shown in FIG. 1e and in a T-S (temperature-entropy) diagram as shown in FIG. 1f. The Stirling cycle is a closed cycle in that the working fluid is typically not replaced during the course of the cycle.
Stirling cycle engines have not generally been used in practical applications, due to several daunting engineering challenges to their development. These involve such practical considerations as efficiency, vibration, lifetime, and cost. The instant invention addresses these considerations.
In accordance with preferred embodiments of the invention, a method is provided for controlling the fuel-air ratio of a burner of an external combustion engine having a heater head, where the burner uses a blower responsive to a blower drive signal for injecting air into the burner. The method is based at least on the concentration of a gas in an exhaust gas product of a combustion chamber of the burner and includes measuring the gas concentration in the exhaust gas product, deriving a gas concentration signal from the measured gas concentration, determining the fuel-air ratio from the gas concentration signal and the sign of the derivative of the gas concentration signal with respect to the blower drive signal, and controlling the fuel-air ratio by adjusting an air flow rate into the burner.
In accordance with another embodiment of the invention, the gas concentration in the exhaust gas product of the burner is measured using a gas composition sensor. The gas composition sensor may be an oxygen sensor or a carbon monoxide sensor. The air flow rate may be adjusted to obtain a predetermined optimal fuel-air ratio, where the optimal fuel-air ratio is based on at least a temperature of the air injected into the combustion chamber of the burner. In one embodiment, the temperature of the air may be measured using a temperature sensor. In another embodiment, the temperature of the air is determined based at least on a temperature of the heater head.
In a further embodiment, the gas composition sensor is a carbon monoxide sensor and the air flow rate into the burner is adjusted to minimize the gas concentration signal produced by the carbon monoxide sensor. Alternatively, the air flow rate may be adjusted to obtain a gas concentration signal from the carbon monoxide sensor that is below a predetermined value.
In accordance with another aspect of the present invention, a system is taught for controlling the fuel-air ratio of a burner of an external combustion engine having a heater head. The system is based at least on the concentration of a gas in an exhaust gas product of a combustion chamber of the burner, and includes a sensor for measuring the gas concentration in the exhaust gas product of the combustion chamber of the burner and for generating a sensor signal. The system also includes a blower governed by a blower signal for injecting air into the burner. The system further includes a controller for receiving the sensor signal from the sensor. The controller adjusts the blower based at least on the sign of the derivative of the sensor signal with respect to the blower drive signal to control the fuel-air ratio in the burner.
In another embodiment, the system includes a gas composition sensor for monitoring the gas concentration in the exhaust gas product of the burner. The gas composition sensor may be an oxygen sensor or a carbon monoxide sensor. The air flow rate may be adjusted to obtain a predetermined optimal fuel-air ratio, where the optimal fuel-air ratio is based on at least a temperature of the air injected into the combustion chamber of the burner. In one embodiment, the temperature of the air may be measured using a temperature sensor. In another embodiment, the temperature of the air is determined based at least on a temperature of the heater head.
In a further embodiment, the gas composition sensor is a carbon monoxide sensor and the air flow rate into the burner is adjusted to minimize the gas concentration signal produced by the carbon monoxide sensor. Alternatively, the air flow rate may be adjusted to obtain a gas concentration signal from the carbon monoxide sensor that is below a predetermined value.
In accordance with another aspect of the invention, a method for controlling the fuel-air ratio of a fuel-air mixture in a burner of an external combustion engine having a heater head includes determining the fuel-air ratio in the burner and determining a temperature of the preheated air used in the fuel-air mixture in the burner. An air flow rate is adjusted to obtain a predetermined fuel-air ratio, where the optimal fuel-air ratio is based on at least the temperature of the preheated air. In one embodiment, the temperature of the preheated air may be measured using a temperature sensor. In another embodiment, the temperature of the preheated air is determined based at least on a temperature of the heater head.
In accordance with yet another aspect of the invention, a method for igniting a fuel-air mixture, having a fuel-air ratio, in a burner includes determining an optimal fuel-air ratio for igniting the fuel air mixture based on at least the temperature of the air used in the fuel-air mixture. The method further includes setting the fuel-air ratio to an initial ignition fuel-air ratio that contains a higher amount of fuel than the optimal fuel-air ratio by adjusting a fuel-flow rate into the burner. The initial ignition fuel-air ratio is maintained until the fuel-air mixture ignites. Once the fuel-air mixture has ignited, the fuel flow rate is reduced to obtain the optimal fuel-air ratio.
In another embodiment, a method for igniting a fuel-air mixture in a burner includes setting the fuel-air ratio of the fuel-air mixture to an ignition fuel-air ratio that is retrieved from a memory area of a controller of the burner and attempting to ignite the fuel-air mixture at the ignition fuel-air ratio. The method further includes adjusting the fuel-air ratio, if the fuel-air mixture does not ignite, by alternately increasing and decreasing the fuel-air ratio above and below the ignition fuel-air ratio until the fuel-air mixture ignites. The fuel-air ratio at which the fuel-air mixture ignites is then stored in the memory area of the controller as the ignition fuel-air ratio. The method may further include, before each adjustment of the fuel-air ratio, purging the burner of unburned fuel-air mixture. In one embodiment, the fuel-air ratio is adjusted by changing the air-flow rate into the burner. In another embodiment, the fuel-air ratio is adjusted by changing a fuel-flow rate into the burner.